Ferdinand Julius Cohn (24 January 1828 – 25 June 1898) was a German biologist.

Cohn was born in Breslau in the Prussian Province of Silesia. At the age of 10 he suffered hearing impairment. He received a degree in botany in 1847 at the age of nineteen at the University of Berlin. He was a teacher and researcher at University of Breslau for his entire career. In the 1850s he mostly studied algae. In the 1860s he studied plant physiology in several different aspects. From 1870 onward he mostly studied bacteria. He published over 150 research reports during his lifetime. The University of Breslau became an innovative center for plant physiology and microbiology while he was there.

Cohn was the first to classify algae as plants, and to define what distinguishes them from green plants. His classification of bacteria into four groups based on shape (sphericals, short rods, threads, and spirals) is still in use today. Among other things Cohn is remembered for being the first to show that Bacillus can change from a vegetative state to an endospore state when subjected to an environment deleterious to the vegetative state.

In 1885 he received the Leeuwenhoek Medal.